In the applied sciences, various fields of study require the analysis of two-dimensional or three-dimensional volume data sets wherein each data set may have multiple attributes representing different physical properties. An attribute, sometimes referred to as a data value, represents a particular physical property of an object within a defined two-dimensional or three-dimensional space. A data value may, for instance, be an 8-byte data word which includes 256 possible values. The location of an attribute is represented by (x, y, data value) or (x, y, z, data value). If the attribute represents pressure at a particular location, then the attribute location may be expressed as (x, y, z, pressure).
In the medical field, a computerized axial topography (CAT) scanner or magnetic resonance imaging (MRI) device is used to produce a picture or diagnostic image of some specific area of a person's body, typically representing the coordinate and a determined attribute. Normally, each attribute within a predetermined location must be imaged separate and apart from another attribute. For example, one attribute representing temperature at a predetermined location is typically imaged separate from another attribute representing pressure at the same location. Thus, the diagnosis of a particular condition based upon these attributes is limited by the ability to display a single attribute at a predetermined location.
Geophysical methods have been used in the search for oil and gas since the late 1800's when the earliest tools used gravity measurements to identify potentially hydrocarbon-bearing rocks. Reflection and refraction-seismic data were first used for exploration in the 1920's. Early interpretation by a geologist or a geophysicist, hereinafter referred to as geoscientists or interpreters, was made by hand-marking on seismograph film and later on large paper “bed sheets.” The process was tedious and time consuming.
Two-dimensional seismic surveys were first created by laying a grid pattern of long cables containing shot points and receivers over large areas, called surveys. Each cable represents a “line.” Shot points emit sound (generated by dynamite or other types of explosions), and receivers, or geophones, record the seismic waves as they reach the receiver. The distance and travel time of the sound waves are recorded at each geophone and graphically represented as seismic wavelets. Originally, only analog data was available, but it was replaced by digital information as the computer age grew. For two-dimensional seismic interpretation, geoscientists made interpretations, based on the character of the wavelet at the actual line and interpolated the information in areas between the lines.
In the 1970's, technological advances allowed the use of three-dimensional seismic. Seismic surveys were designed as dense grids, and data could be represented as a three-dimensional volume or cube. Computer improvements in the 1980's made three-dimensional seismic interpretation on a workstation possible. Early three-dimensional seismic interpretation only permitted geoscientists to observe and interpret data on either vertical sections or horizontal slices.
In the oil and gas industry, three-dimensional seismic-data sets (3D seismic data) are comprised of regularly spaced orthogonal volumes of data samples. The data is displayed in two-dimensional planes as either vertical seismic sections or horizontal seismic slices. In turn, the vertical seismic sections are often displayed in connected, flat planes forming an interlocking “fence” that cuts through a three-dimensional geological region of interest. Interpreters study multiple seismic displays to interpret the location and nature of geological structures and stratigraphic boundaries and to plan well locations.
Resulting from the accelerated processing capabilities of modern computers, the use of dense three-dimensional seismic data has surpassed the use of two-dimensional seismic in petroleum exploration. Volume visualization has also become a fundamental feature in mainstream interpretation applications. Typically, applications present seismic volumes as slices, fences, shell cubes, and translucent cubes by using different volume rendering techniques. The challenges of volume visualization are mainly speed and size. Today's standard computer provides greater computation power than yesterday's super computer. 64-bit computers easily load several gigabytes of data into memory, and computer clusters push volume sizes ever larger.
This accelerated computation power now makes visualization of multiple seismic volumes possible. Visualizing multiple raw volumes and attribute volumes can increase understanding of their relationships and improve interpretation of oil and gas prospects. The importance of the combination of multiple volumes is well known.
Today's workstations and visualization technology let interpreters see data in a truly realistic, three-dimensional environment, more closely resembling the actual rocks beneath the Earth's surface. With three-dimensional volumes, the subsurface seismic wave field is closely sampled in every direction, resulting in more accurate structural and stratigraphic interpretation. Computers allow the seismic volume to display as voxels, or volume elements, that may be rendered with differing degrees of opacity and may be calculated using specific attribute algorithms.
The seismic data is collected and processed to produce three-dimensional volume data sets comprising “voxels” or volume elements, whereby each voxel may be identified by the x, y, z coordinates of one of its eight corners or its center. Each voxel also represents a numeric data value (attribute) associated with some measured or calculated physical property at a particular location. Examples of geological seismic data values include amplitude, phase, frequency, and semblance. Different data values are stored in different three-dimensional volume data sets, wherein each three-dimensional volume data set represents a different data value. When multitude data sets are used, the data value for each of the data sets may represent a different physical parameter or attribute for the same geographic space. By way of example, a plurality of data sets could include a seismic volume, a temperature volume and a water-saturation volume. The voxels in the seismic volume can be expressed in the form (x, y, z, seismic amplitude). The voxels in the temperature volume can be expressed in the form (x, y, z, ° C.). The voxels in the water-saturation volume can be expressed in the form (x, y, z, % saturation). The physical or geographic space defined by the voxels in each of these volumes is the same. However, for any specific spatial location (xo, yo, zo), the seismic amplitude would be contained in the seismic volume, the temperature in the temperature volume and the water-saturation in the water-saturation volume. In order to analyze certain sub-surface geological structures, sometimes referred to as “features” or “events,” information from different three-dimensional volume data sets may be separately imaged in order to analyze the feature or event.
Geoscientists examine the seismic data to identify continuous reflections, which often represent horizons, and discontinuities in these reflections, which represent faults or other structural components capable of trapping hydrocarbons. Anomalies, such as a “bright spot,” in horizons frequently indicate the presence of oil or gas.
Software technology permits interpreters to use a variety of industry-standard algorithms to calculate attributes on seismic volumes. The purpose is to extract information about a seismic horizon that might identify hydrocarbons. An attribute, for example, may contain time, amplitude, frequency, and attenuation information for the seismic data. Geoscientists select algorithms and make calculations over the seismic data to reveal areas of interest that would otherwise remain obscured. Some of the common attribute calculations measure frequency, phase, azimuth, dip-azimuth, and edge detection. Bandpass filters allow only selected frequencies to pass through a calculation window. More recently, algorithms have also measured spectral decomposition of the seismic data.
The use of multiple volumes in visualization is one of the leading trends for hydrocarbon exploration and production operations. Visualization can incorporate data from multiple three-dimensional surveys and time lapse four-dimensional seismic surveys into a single display. There are several approaches for visualizing multiple three-dimensional volume data sets. The simplest approach is to show corresponding displays from each dataset in separate, linked views with coupled cursors. Another approach is to combine multiple data sets into a single display. As such, coherency and amplitude volumes can be combined using bump mapping, in which the hue of each pixel is controlled by the reflection amplitude and the lighting (bump effect) is controlled by the coherency value. Data animation can show four-dimensional time-lapse sequences; this technique is especially effective for reservoir simulation results in which the changes in the seismic data, the reservoir fluids, and the well logs are compared over time.
Traditionally, software displayed each volume in a separate window. This approach makes it difficult to analyze the coherence and relationship between volumes. Using overlays and combinations of volumes makes interpreting the data much easier.
Methods are known in the art regarding how to calculate, manage, and interpret attribute volumes using volume-based techniques, which allow interpreters to quantitatively estimate rock and fluid properties for a reservoir. Combining multiple volumes for oil and gas data visualization and using multiple attributes and disciplines in the visualization process helps geoscientists to classify reservoirs based on empirical correlation to geologic and petrophysical information.
Modern software applications allow simultaneous use of multiple seismic volumes. For instance, an interpreter may look at a standard time domain seismic volume while observing the same data in a discontinuity volume. The concept of multiple volume interpretation lets the geoscientist rapidly interpret large areas of data with far greater accuracy and speed.
The “stencil” paradigm in two-dimensional painting programs to combine multiple volumes has been previously used. Three possible ways, including RGBA color, opacity, and intensity, are used to define transfer function. Each volume can be associated with one of these three types of transfer functions, and the layers are combined at the fragment level. Each layer's transfer function is pre-integrated independently and composite.
In addition to oil and gas exploration, other fields, in particular medicine, have greatly contributed to multi-volume rendering research. Radiotherapy treatment planning involves three volumes: a Computed tomography (CT) volume, a Dose volume, and a Segmented Object volume. Ray Casting direction volume rendering is then applied. Three levels of data intermixing have been defined: image level, accumulation, and illumination intermixing. The combination of position emission tomography (PET), CT, and magnetic resonance imaging (MRI) medical volumes have previously been used. Ray casting volume rendering was also used. Such applications combine cutting techniques and data-intermixing techniques.
In the past decade, the three-dimensional graphics computational power and on-board memory in off-the-shelf graphics cards have sustained rapid growth. The programmability of the graphics processing unit (GPU) opens up new possibilities to move some of the central processing unit (CPU) algorithms to the GPU for improved performance and quality. For example, U.S. Pat. No. 7,298,376 (the “'376 patent”), which is assigned to Landmark Graphics Corporation and incorporated herein by reference, uses a graphics card to co-render multiple attribute volumes in real time as an enhanced image. Nevertheless, this technique is limited by the specific algorithm(s) used to perform pixel intermixing by bump mapping.
Other techniques have been developed in this field for imaging multiple three-dimensional volume data sets in a single display, however, not without considerable limitations. One example includes the technique published in The Leading Edge called “Constructing Faults from Seed Picks by Voxel Tracking” by Jack Lees. This technique combines two three-dimensional volume data sets in a single display, thereby restricting each original 256-value attribute to 128 values of the full 256-value range. The resolution of the display is, therefore, significantly reduced, thereby limiting the ability to distinguish certain events or features from the rest of the data. Another conventional method combines the display of two three-dimensional volume data sets, containing two different attributes, by making some data values more transparent than others. This technique becomes untenable when more than two attributes are combined.
Another technique used to combine two different three-dimensional volume data sets in the same image is illustrated in U.S. Pat. No. 6,690,820, which is assigned to Landmark Graphics Corporation and incorporated herein by reference. This patent describes a technique for combining a first three-dimensional volume data set representing a first attribute and a second three-dimensional volume data set representing a second attribute in a single enhanced three-dimensional volume data set by comparing each of the first and second attribute data values with a preselected data value range or criteria. For each data value where the criteria are met, a first selected data value is inserted at a position corresponding with the respective data value in the enhanced three-dimensional volume data set. For each data value where the criteria are not met, a second selected data value is inserted at a position corresponding with the respective data value in the enhanced three-dimensional volume data set. The first selected data value may be related to the first attribute and the second selected data value may be related to the second attribute. The resulting image is an enhanced three-dimensional volume data set comprising a combination or hybrid of the original first three-dimensional volume data set and the second three-dimensional volume data set. As a result, the extra processing step needed to generate the enhanced three-dimensional volume data set causes interpretation delays and performance slow downs. Furthermore, this pre-processing technique is compromised by a “lossy” effect which compromises data from one seismic attribute in order to image another seismic attribute. Consequently, there is a significant loss of data visualization.
There is therefore, a need for alternative techniques to image multiple three-dimensional volume data sets in real time as a final combined image, which are not limited by a specific algorithm for intermixing voxels, pixels and/or images.